2.1 Study design and setting
This retrospective study was conducted using data provided by JMDC Inc. (Tokyo, Japan), a commercial healthcare data vendor [11,12]. The JMDC database includes data from >100 society-managed health insurance plans covering employees and their dependents inclusive of children. The cumulative number of enrollees since 2005 is approximately 5 million. The study period was between December 1, 2018 to March 31, 2019. In Japan, during the 2018/2019 influenza season, the dominant circulating virus was A (H3N2) strain (56.0%), followed by A (H1N1) strain (36.5%) and all types of B strains (7.5%) [13]; this represented lower influenza B activity than had been observed in the preceding seasons.
This study protocol was approved by the ethics committee at Kyoto University, including waiver of informed consent from each patient.